If you untie a rope, it no longer appears into your inventory. It is stolen by dirty greasemonkeys instead.
I had this happen as well, didn't report it because I wasn't sure if it was a glitch I caused by moving too soon after untying it and canceling the actions or something else, and nobody else seemed to have the problem. It was a rope given to me by a new character, so I think that it is ropes purchased in the start area or in the trade hall. I will have to check with the player to be sure.